1. Field of the Invention
The present invention relates to a method of integrated circuit manufacture. More specifically, the invention relates to a method of combining a plurality of semiconductor regions each defined by a reticle image.
2. Description of the Related Art
Integrated circuits are typically fabricated using photolithography techniques to produce a desired pattern from a photographic mask on a substrate material prior to a processing setup such as an etching step. Prior to photolithographic masking, a semiconductor wafer is cleaned, then covered with a solution of photoresist by spincoating, spraying or immersion. The solution is allowed to dry and then exposed to light or near ultraviolet radiation through the mask.
A photolithographic system for selectively irradiating a semiconductor wafer includes a radiation source, a lens or mirror and a mask or reticle. Photographic masks and photographic reticles are used to selectively pattern a semiconductor wafer. The mask and reticle differ in that a mask transfers a pattern onto an entire wafer in a single exposure. A reticle transfers a pattern onto only a portion of the wafer. In a photolithographic system that employs projection printing, the radiation source illuminates through the mask or reticle to the lens or mirror, and the lens or mirror focuses an image of the mask or reticle onto the photoresist coating of the semiconductor wafer.
One projection printing technique employs a projection scanner to transfer a pattern from a mask or reticle to a semiconductor wafer. The projection scanner uses a reflective spherical mirror to project an image onto a wafer by scanning the wafer and the mask with a narrow arc of radiation.
Another projection printing technique uses a step and repeat system, which is also called a stepper, to project an reticle image only onto a portion of the wafer. Multiple images of the reticle pattern are stepped and repeated over the entire wafer using multiple exposures. The reticle pattern is typically reduced 2.times. to 10.times. by the lens to form a small size but high-resolution image on the wafer surface, although non-reduction (1.times.) lens are available to cover a larger field on the wafer.
A photolithographic system uses an illumination source, such as a mercury-vapor lamp, to transfer a pattern to a wafer. A mercury-vapor lamp creates a discharge arc of high-pressure mercury vapor and emits a characteristic spectrum that contains several sharp lines in the ultraviolet region including the I-line at a wavelength of 365 nm, the H-line at a 405 nm wavelength and the G-line at a 436 nm wavelength. Generally photolithographic systems operate using the either the G-line, the I-line, a combination of the lines, or in the deep UV wavelengths of around 240 nm. A suitable illumination is attained using high power mercury-vapor lamps which draw 200 to 1,000 watts and generate an ultraviolet intensity of approximately 100 milliwatts/cm.sup.2.
A typical reticle is constructed from glass with relatively defect-free surfaces and a high optical transmission at the radiation wavelength. Reticle glasses include soda-lime glass, borosilicate glass, and quartz. Quartz advantageously has a low thermal expansion coefficient and high transmission for near and deep ultraviolet light.
The term resolution refers to the ability of an optical system to distinguish closely spaced objects. The minimum resolution of a photolithographic system is the dimension of minimum linewidth or space that the system adequately prints or resolves. Optical photolithography currently attains a resolution of 0.35.mu. or less. Feature sizes approach 0.25.mu. and below with the features extending across wafer areas of a square inch and more. To improve resolution various alternative technologies are under development, including electron-beam, ion-beam, and x-ray technologies. These alternative technologies have achieved patterning capabilities that exceed limits of optical systems. Electron-beams and ion-beams can also directly write image patterns onto the photoresist without the use of a mask or reticle, for instance by using a controlled stage to position the wafer beneath the tool. However, these alternative approaches have certain drawbacks. For instance, electron-beam lithography has low throughput, x-ray lithography has difficulties with fabricating suitable masks, and ion-beam lithography has low throughput and difficulties with obtaining reliable ion sources.
One problem that arises with imaging of a wafer using a plurality of reticle images is the difficulty of achieving a suitable registration between the image fields on the semiconductor wafer. In particular, structures in different reticle fields are typically connected by overlapping of continuous lines that span the exposure fields of several reticles. Errors in registration can cause a connecting line between exposure fields to become laterally displaced and, therefore, disconnected. For electrically conductive structures, such as polysilicon and conductive metals, that are intended to form a conductive loop in a continuous circuit, a disconnection between structures constitutes an open circuit. For an electrically conductive loop having a registration error that does not result in a disconnected but rather a significantly attenuated line width, the line resistance may be substantially elevated, thereby impacting the performance of the circuit. In addition, a metal line such as an aluminum line that is significantly narrowed may become susceptible to high resistance or open lines due to electromigration. Disconnected lines and narrowed lines caused by misregistration between reticle image fields are typically stitched by depositing metal contacts over the ends of line segments. This approach disadvantageously requires additional processing steps for depositing and etching the metal contacts.
Another problem arising with fabrication of circuits by imaging using reticle image fields is that substantial silicon area is lost at the boundaries between reticle imaging fields.
What is needed is a technique for stitching line segments defined by adjacent reticle image patterns projected onto a photoresist layer overlying a semiconductor wafer so that segments are suitably interconnected.